This invention relates to a position transducer comprised of at least one sensor having an output that is based upon differential reactance.
Of the known position feedback transducers, optical encoders have severe drawbacks, including sensitivity to temperature and mechanical shock, difficult mounting and alignment requirements, and cable length limitations; optical encoders are particularly disadvantageous in electrically noisy environments. Although induction-based electromagnetic feedback transducers are generally reliable and accurate, they afford only single speed capability, typically with low electrical frequency, and they require relatively complex interfaces; such transducers are also sensitive to magnetically harsh and electrically noisy environments, they are subject to mechanical run-out, mounting and concentricity alignment are difficult, and they require integral transformers for contactless transfer of AC power to the rotor.
Non-permanent magnet reluctance-based transducers take advantage of variations in magnetic path reluctance, which occur when two ferromagnetic members slide or rotate past one another. In one known system, the difference of two reactances is measured to produce a sinusoidal output that has low harmonic content and distortion, and in which the frequency (number of electrical cycles per mechanical revolution) is relatively high. Disadvantages of devices of this kind include however single speed or frequency capability, sensitivity to magnetic disturbance fields, difficulty of mounting and concentricity establishment, and run-out tendencies.
In permanent magnet reluctance sensors, changes in path reluctance, produced by relative movement of ferromagnetic members, are used to modulate magnetic flux in stator poles. Despite knowledge of the principle, permanent magnets have seldom been used heretofore in position sensors; that is so primarily because voltages induced in the sensing coils by the magnetic flux field make the acquisition of accurate, high-resolution position information most difficult to achieve.
In U.S. Pat. No. 4,406,983, Ramirez discloses a rotational magnetic transducer for commutating a motor, which transducer affords tolerance to stray magnetic fields and a capability for operation in hostile environments. The device employs two coils per channel, so arranged that the permanent magnet-induced back-EMFs cancel one another. Output is determined by the total inductance of both coils, which contribute equally to the signal at all times and in all rotor positions.
Stacy U.S. Pat. No. 5,140,245 also provides a permanent magnet-based transducer that is suited for operation in hostile environments. The device is a three-phase, permanent magnet-excited voltage generator, which has a relatively low electrical frequency to enable commutation of a brushless DC/AC servo motor or other synchronous motor. The output from each of the three channels is a signal (a permanent magnet flux-induced voltage) having a magnitude that is directly related to rotational speed, the integral value of the signal therefore being indicative of rotational position.
A so-called "permanent magnet resolver" is disclosed in Carlen, U.S. Pat. No. 5,160,886, and is described as being based upon the use of two coils having back-EMF effects that are shifted 180.degree. electrical from one another. The device is a two coil per channel, single reactance sensor, in which one electronic detector per coil is employed.